A chalcopyrite semiconductor thin film comprising Group I, III, and VI elements which are used for an absorption layer of solar cells has a high absorption coefficient and is available for photovoltaic applications to solar cells. The chalcopyrite semiconductor thin films, such as Cu(In,Ga)Se.sub.2 thin film, are formed by the Bi-layer method, namely; a chalcopyrite layer containing excessive Cu of Group I elements are formed, thereon a chalcopyrite layer containing excessive Group III elements (In,Ga) is formed. As a result, the crystal grain size becomes bigger. Moreover, the Bi-layer method is also used so as not to precipitate a different phase compound such as Cu.sub.2-x Se. However, according to the method, many component elements are evaporated at the same time during forming films, it is difficult to control the composition of Cu(In, Ga)Se.sub.2 thin film, and reproducibility of films is not so good. Cu(In,Ga)Se.sub.2 thin film formed by this method has low adhesion to the under-part of an electrode, which has been a problem in the practical application of solar cells.
As a means of solving the above mentioned problem, Contreras et al of National Renewable Energy Laboratory(NREL), proposed a method of forming Cu(In,Ga)Se.sub.2 thin film at the 1st World Conference on Photovoltaic Energy Conversion in December 1994. They proposed the 3-Stage method. According to the 3-Stage method, Cu.sub.2 Se is evaporated on (In,Ga).sub.2 Se.sub.3 precursor film of the first layer until the thin films have a Cu-rich composition, thereon (In,Ga).sub.2 Se.sub.3 of the third layer is evaporated so as to form again a (In, Ga)-rich composition. (The term "rich" is same meaning as "excessive".) According to this method, since a few component elements are used during forming films, the reproducibility of thin films containing optional composition is good.
When forming the 2-Stage Cu(In,Ga)Se.sub.2 thin films which have the same composition as 3-Stage thin films by applying the 3-Stage method proposed by NREL, the crystal grain size of the 2-stage Cu(In,Ga)Se.sub.2 film is small compared with 3-stage Cu(In,Ga)Se.sub.2 film and the conversion efficiency is low when the films are used in solar cells. Therefore, in the 3-Stage method, when evaporating Cu.sub.2 Se of the second layer, Cu and Se must be evaporated until the films are Cu-rich. During this process, it is difficult to predict the final composition of films after (In,Ga).sub.2 Se.sub.3 of the third layer is evaporated unless the composition is recognized during forming films. If the final composition of the thin film is Cu-rich, a different phase compound such as Cu.sub.2-x Se will consequently precipitate and deteriorate the properties of solar cells. Moreover, though a condition of flux from various source evaporation is once established by experiments in order to make a proper composition of Cu, it is difficult to reproduce films of the same composition due to a subtle difference of degree of vacuum condition or types of residual gas. Therefore, the problems in forming Cu(In,Ga)Se.sub.2 thin film are: to control Cu(In, Ga)Se.sub.2 thin film to be a proper Cu-rich composition with good reproducibility in depositing Cu.sub.2 Se of the second layer; and to control films to be a proper (In,Ga)-rich composition with good reproducibility in depositing (In,Ga).sub.2 Se.sub.3 of the third layer.
In addition, when thin films are Se-deficient in the process of forming the Cu(In,Ga)Se.sub.2 thin films, the films tend to have a metallic property, and such Se-deficient films can not function as an absorption layer of solar cells.
An object of the present invention is to solve the prior problem and to provide a method and an apparatus of manufacturing the chalcopyrite semiconductor thin films whose composition can easily be controlled and reproducibility is good.